When new cellular technologies are introduced in cellular operators' network, there is often a desire to minimise the number of additional antennas and feeders. When the global system for mobile telecommunications (GSM) was introduced in USA on existing time division multiple access (TDMA) sites, co-siting solutions minimising the number of additional antennas and feeders were developed.
FIG. 1 shows a known arrangement with a TDMA radio base station TDMA RBS, with two RX ports RXA, RXB and a TX port. The arrangement in FIG. 1 can be a standalone TDMA 1900 MHz installation with tower mounted amplifiers TMA, which are powered with DC voltage overlaid on the feeder, for optimised receiver sensitivity.
FIG. 2 shows a further example of a known arrangement. By reusing the TDMA receive (RX) antennas as transmit/receive (TX/RX) antennas for GSM, and forwarding receive signals from a GSM radio base station to a TDMA radio base station, additional antennas and feeders are avoided. As shown in FIG. 2, this is done by using a GSM radio base station GSM RBS, equipped with an antenna sharing unit ASU, as a master base station. The antenna sharing unit ASU extracts and forwards amplified receive signals to the slave TDMA radio base station TDMA RBS, via two co-siting ports RXB2, RXA2 of the master GSM radio base station.
FIG. 3 shows a known solution to introduce wide band code divisional multiple access (WCDMA) into existing sites. Two transmit/receive TX/RX ports of a WCDMA radio base station WCDMA RBS are connected to a respective master port of a respective diplex duplex unit DDU, which is a form of combiner. In addition to the master port, each diplex duplex unit has an antenna port ANT and a receive port RX and a slave port. By means of the diplex duplex units DDU, it is possible, in the arrangement in FIG. 3, to reduce the number of antenna feeders F by two. Each diplex duplex unit forwards receive signals from the antenna port to the master port, forwards receive signals from the receive port RX to the slave port, and combines transmit TX signals on the master and slave ports to the common antenna port ANT. The WCDMA radio base station is provided with two co-siting ports RXB1, RXA1, each connected to the respective receive port RX of the respective diplex duplex unit DDU. The slave port of the respective diplex duplex unit DDU is connected to a respective transmit/receive TX/RX port of a GSM base station GSM RBS. Similar to the antenna sharing unit ASU in the GSM base station in FIG. 2, the WCDMA radio base station is equipped with functions for extracting receive signals and forwarding them to the GSM base station, via the diplex duplex units DDU. The GSM base station is provided with an antenna sharing unit ASU and a TMA simulator TMAS, extracts and forwards amplified receive signals to a slave TDMA radio base station TDMA RBS. Thus, the GSM radio base station will be a slave to the WCDMA base station, and the TDMA base station will in turn be slave to the slave GSM base station.
A main drawback with the solution shown in FIG. 3 is that a receiver of the TDMA base station will be last in a quite long chain with a series of successive alternating amplifications and attentuations, which affects the TDMA receiver sensitivity negatively. More specifically, an TDMA receive signal path will pass the antenna, the tower mounted amplifier TMA, which amplifies the signal, the diplex duplex unit feeder, which attenuates the signal, the WCDMA base station (amplification), a cable (attenuation), the GSM base station (amplification), and again a cable (attenuation), to arrive at the TDMA receiver. Thus, the known solution in FIG. 3 provides a cascade of amplifier stages, and the problem with such cascading can easily be demonstrated by Friis formula:
      NR    TOT    =            NR      1        +                            NR          2                -        1                    g        1              +                            NR          3                -        1                              g          1                ×                  g          2                      +                            NR          4                -        1                              g          1                ×                  g          2                ×                  g          3                      +    …    +                            NR          n                -        1                              g          1                ×                  g          2                ×                  g          3                ×        …        ×                  g                      n            -            1                              whereNRTOT=total Noise Ratio,NR1, NR2, etc.=Noise Ratio for each stage, andg1, g2, etc.=Gain Ratio for each stage.